This application claims benefit of priority under 35USC xc2xa7119 to Japanese Patent Application Nos. 2000-281353 and 2001-208028 filed on Sep. 18, 2000 and Jul. 9, 2001, respectively, in Japan, the entire contents of which are incorporated by reference herein.
The present invention relates to a method of forming a polycrystalline semiconductor film. Particularly, this invention relates to a method of forming a polycrystalline semiconductor film as an active layer for thin-film transistors used for liquid crystal displays, etc.
With advanced miniaturization in liquid crystal displays, thin-film transistors (TFT) having polycrystalline silicon with high mobility used for active layers have recently been used instead of known transistors having amorphous silicon as active layers.
Described with reference to FIG. 6 is a known method of producing a transistor having polycrystalline silicon used for an active layer.
As shown in FIG. 6(a), a thin film 42 of amorphous semiconductor is deposited on an insulating film 41 made of glass for example. The thin film 42 is irradiated with energy-rich beams such as the excimer laser to be melted for recrystallization, and hence changed into a polycrystalline thin film 43 as shown in FIG. 6(b). The thin film 43 is patterned, followed by impurity doping to form semiconductor films 43a of low concentration as shown in FIG. 6(c).
Next, as shown in FIG. 6(d), the semiconductor films 43a are covered with a gate insulating film 44. A metallic film is formed on the film 44 and pattered, thus forming a gate electrode 45 for an n-channel transistor and a metallic film 45a that covers the semiconductor film 43a for a p-channel transistor. Subsequently, p- or n-type dopant is implanted into the semiconductor films 43a at a high concentration via the gate electrode 45 as a mask, to form an n-type source/drain region 46 as shown in FIG. 6(d).
A resist pattern 50 made of photoresist is formed as shown in FIG. 6(e). The metallic film 45a is patterned with this resist pattern to form a gate electrode 45a for the p-channel transistor. P-type dopant of high concentration is implanted into the semiconductor film 43a for the p-channel transistor while masked with the resist pattern 50 and the gate electrodes 45a to form p-type source/drain regions 47.
The resist pattern 50 is removed, followed by annealing for dopant activation, and an interlayer insulating film 48 is formed on the entire surface as shown in FIG. 6(f). Contact holes are formed on insulating films 48 and 44 and are filled with an electrode-material film. The film is patterned to form source/drain electrodes 49, thus producing the transistor.
Not only by irradiation of energy-rich beams such as the excimer laser, polycrystalline silicon films can be formed by, for example, solid phase epitaxy with annealing amorphous silicon for a long time at a temperature in the range from about 400xc2x0 C. to 600xc2x0 C. In general, however, polycrystalline silicon films formed by solid phase epitaxy have lower carrier mobility than those formed by irradiation of energy-rich beams for recrystallization by melting. Moreover, large substrates can easily be recrystallized by irradiation of energy-rich line beams, but not by solid phase epitaxy. Polycrystalline silicon films formed by solid phase epitaxy thus cannot be used for high-speed circuitry, and hence used only for small liquid crystal displays.
Irradiation of energy-rich beams such as the excimer laser for recrystallization of amorphous silicon by melting causes roughness on the silicon surface. Shown in FIG. 5 is a cross-sectional view of a thin-film transistor having an active layer of polycrystalline silicon formed by irradiation of the excimer laser. It is expected from FIG. 5 that a gate insulating film 44, which is formed on a polycrystalline silicon 43a having roughness formed on an insulative substrate 41, has thin sections formed on convex portions of the polycrystalline silicon.
Gate insulating films having thin portions cause a decrease in gate dielectric strength. FIG. 4 indicates leak current dependency on voltage for oxide films. One oxide film was formed on a substrate of single crystalline silicon whereas the other film having the same thickness as the former was formed a polycrystalline silicon film which was formed by irradiating the excimer laser. The graphs teach that a leak current flowed through the oxide film formed on the polycrystalline silicon film at an extremely low voltage compared to the single crystal silicon. It is speculated that electric fields converged on the surface convex sections might have caused generation of current on the convex sections.
Thin-film transistors having polycrystalline silicon as active layers thus require a thick gate insulating film. An on state current for thin-film transistors are inverse proportional to the thickness of a gate insulating film. A thick gate insulating film mitigates roughness on the surface of a polycrystalline silicon film, however, it causes decrease in performance of transistors.
It is speculated that such roughness that causes decrease in performance of thin-film transistors is formed on the surface of a polycrystalline silicon due to segregation of an oxide film formed on the surface of amorphous silicon or formed with oxygen existing in the atmosphere irradiated with laser, during recrystallization by melting.
Such roughness thus can be mitigated by complete removal of an oxide film formed on the surface of amorphous silicon with lowering a partial pressure of oxygen existing in the atmosphere irradiated with laser. Laser annealing under this condition, however, causes abrasion of a silicon film at energy lower than that of laser beam irradiation required for grain of polycrystalline silicon to become large enough. Or, it causes increase in energy of laser beams for crystallization. This results in decreasing in operating rate of an apparatus for irradiating the laser beams.
Such a problem is solved by atomospheric laser annealing for enlarging grain of polycrystalline silicon, followed by removal of the surface oxide film with hydrogen fluoride (HF) and vacuum laser annealing for minimizing roughness on the surface of polycrystalline silicon. This is proposed in K. Suga et al., xe2x80x9cThe Effect of a Laser Annealing Ambient on the Morphology and TFT Performance of Poly-Si Filmsxe2x80x9d, Society for Information Display 00 DIGEST, p534-537, May 2000.
This method, however, requires decompression from ambient pressure to vacuum for laser annealing or two separate systems for ambient pressure only and vacuum only. Either way requires HF treatment after atomospheric laser annealing, thus decreasing productivity.
A purpose of the present invention is to provide a method of forming a polycrystalline semiconductor film that provides high performance for thin-film transistors having this polycrystalline semiconductor film and high productivity.
A first aspect of a method of forming a polycrystalline semiconductor film according to the present invention includes depositing an amorphous semiconductor film on a substrate, a first irradiating the amorphous semiconductor film with an energy-rich beam in an atmosphere of a gas containing an inert gas as a major component with a specific amount of oxygen, to change the amorphous semiconductor film into a polycrystalline semiconductor film, and a second irradiating the polycrystalline semiconductor film with an energy-rich beam in an atmosphere of a gas containing an inert gas as a major component with oxygen of an amount less than the specific amount.
The amount of oxygen for the first irradiating may be 5 ppm or more but less than 10% and the amount of oxygen for the second irradiating is preferably 200 ppm.
The total amount of energy for the second irradiating may be larger than the total amount of energy for the first irradiating.
It may be that the polycrystalline semiconductor film is exposed to an atmosphere between the first irradiating and the second irradiating, the atmosphere having an oxygen concentration higher than oxygen concentrations in the atmospheres in the first and the second irradiating.
The first and the second irradiating may be performed in the same processing chamber.
It may be to perform removing a natural oxide film formed on the surface of the amorphous semiconductor film before the first irradiating.
It may be to perform crystallization while splaying an area of the substrate, that is irradiated with the energy-rich beam, with a gas at a pressure higher than an ambient pressure in a processing chamber.
It may be to use an excimer laser beam as the energy-rich beam.
The number of irradiation of the energy-rich beam in the first irradiating may be smaller than the number of irradiation of the energy-rich beam in the second irradiating.
Moreover, a second aspect of a method of forming a polycrystalline semiconductor film according to the present invention includes forming a polycrystalline semiconductor film on a substrate, irradiating the polycrystalline semiconductor film with an energy-rich beam in an atmosphere of a gas containing an inert gas as a major component with oxygen, an amount of the oxygen being 1 ppm or more but 50 ppm or less.
Furthermore, a third aspect of a method of forming a polycrystalline semiconductor film according to the present invention includes depositing an amorphous semiconductor film on a substrate, removing a natural oxide film on the amorphous semiconductor film, first irradiating the amorphous semiconductor film with an energy-rich beam in an atmosphere of a gas containing an inert gas as a major component with oxygen, an amount of the oxygen being 5 ppm or more but less than 10%, to change the amorphous semiconductor film into a polycrystalline semiconductor film, and second irradiating the polycrystalline semiconductor film with an energy-rich beam in an atmosphere containing an inert gas as a major component with oxygen, an amount of the oxygen being 1 ppm or more but 50 ppm or less.
It may be to perform crystallization while splaying an area of the substrate, that is irradiated with the energy-rich beam, with a gas at a pressure higher than an ambient pressure in a processing chamber.